Air-conditioning equipment accounts for the largest proportion of energy consumed by ordinary office buildings and other architectural structures (approximately 38%). Since 30% to 40% of that amount cannot be ventilated from the outside, reducing the loss of air-conditioning energy attributable to ventilation can be said to be an important issue for conserving energy in office buildings. In addition, so-called “sick house syndrome”, which is caused by volatile organic compounds dissipated by building materials, household items and the like, is also becoming a problem. Examples of the causes of these problems include increased airtightness of buildings, increased difficulty in circulating air due to the proliferation of air-conditioners, and ease of accumulation of volatile organic compounds within buildings. In view of these circumstances, buildings in Japan are required to install ventilation equipment in accordance with the revised Building Standards Act enacted in July 2003. In addition, attempts have also been made to add ventilation functions to home air-conditioners, and such efforts have not been limited to Japan, with ventilation of buildings being promoted worldwide.
However, when attempts are made to promote building ventilation, it becomes difficult to maintain building heating even if air-conditioning is employed, thereby resulting in excessively high energy consumption. Consequently, attention is being focused on total heat exchangers that are capable of reducing energy consumption by making it difficult for heat or cold to be released to the outside even if ventilation is employed.
Examples of these energy recovery ventilators include rotary-type energy recovery ventilators that recover heat from exhaust air to intake air by the rotation of a hygroscopic rotor, and a static-type energy recovery ventilators as shown in FIG. 1. In this static-type (fixed-type) energy recovery ventilators, a gas-impermeable energy recovery ventilation element arranged in a corrugated shape allows sensible heat to migrate while dividing into fresh outside supplied air that has been exchanged by ventilation and contaminated interior discharged air, while simultaneously allowing latent heat retaining water to pass from the discharged air to the supplied air by allowing permeation of humidity, thereby reducing the release of heat or cold to the outside.
Since energy recovery ventilation sheets used in the energy recovery ventilation elements of static-type energy recovery ventilator not only allow the migration of sensible heat, but also allow the migration of latent heat by allowing the passage of moist air, energy recovery effectiveness increases. Examples of such sheets include energy recovery ventilation sheets using flame retardant paper made of Japanese paper or pulp, glass fiber mixed paper, inorganic powder-containing mixed paper, and microporous film, paper and nonwoven fabric composed of synthetic polymers. However, since air also ends up penetrating the sheet in the case of using ordinary microporous film, paper or nonwoven fabric, energy recovery ventilation sheets have been reported that have undergone surface treatment such as applying a polymer coating to the surface thereof. For example, Patent Document 1 (Japanese Unexamined Patent Publication No. H6-194093) reports an energy recovery ventilation sheet obtained by coating a polyurethane-based resin containing oxyethylene groups onto a porous sheet that uses polytetrafluoroethylene for the material thereof, while Patent Document 2 (Japanese Unexamined Patent Publication No. 2003-287387) reports an energy recovery ventilation sheet composed of polyester and polyethylene or polypropylene. In both these cases, since a hydrophobic polymer is used for the base material, adequate moisture permeability is unable to be obtained, and as a result thereof, sufficient thermal conductivity is unable to be obtained for use as an energy recovery ventilation sheet.
Patent Document 3 (Japanese Unexamined Patent Publication No, 2008-14623) describes an energy recovery ventilation filter obtained by coating viscose onto hydrophilic fibers in the form of a rayon pulp nonwoven fabric. Coating with hydrophilic rayon fibers is reported to allow the production of a sheet having moisture permeability of 6900 g/m2/24 hours or more and a permeability resistance of 10000 s/100 ml or more. However, due to the inadequate moisture permeability, sufficient performance for use as an energy recovery ventilation sheet is unable to be obtained.
On the other hand, the inventors of the present invention reported a nonwoven fabric structure containing a layer of fine cellulose composed of fine cellulose fibers in Patent Document 4 (Japanese Unexamined Patent Publication No. 2010-115574). However, the multilayered nonwoven fabric structure described in Patent Document 4 was not suitable for use as an energy recovery ventilation sheet due to high air permeability (permeability resistance of 2000 s/100 ml or less).